FIG. 5 is a sectional view showing a configuration of a loudspeaker; and FIGS. 6A and 6B are a plan view and a sectional view showing a configuration of a damper used in the loudspeaker. With reference to FIGS. 5, 6A and 6B, this loudspeaker is described.
Magnetic circuit 4 including annular magnetic gap 4a is configured by bottom plate 1 having a convex-shaped cross section, ring magnet 2 provided on bottom plate 1, and ring upper plate 3 provided on magnet 2. Such a configuration is referred to as an outer magnet type.
Frame 5 is coupled to upper plate 3. The outer peripheral portion of diaphragm 6 is coupled to frame 5. Voice coil 7 is movably disposed in magnetic gap 4a. As shown in FIG. 5, bobbin 7a on which a coil of voice coil 7 is wound extends to the side of diaphragm 6 and coupled to the inner peripheral portion of diaphragm 6. The inner peripheral portion of damper 8 is coupled to bobbin 7a and the outer peripheral portion of damper 8 is coupled to frame 5. On the central part of the upper surface of diaphragm 6, dust cap 9 for preventing entering of dust is provided.
Furthermore, damper 8 is configured in a concentric circular corrugation form spreading on a surface in order to elastically support voice coil 7 via diaphragm 6 and bobbin 7a. Damper 8 is required to have basic performances of being excellent in retaining stability of voice coil 7 and allowing amplitude motion faithfully responding to stress generated in voice coil 7.
Damper 8 is manufactured by a manufacturing process shown in FIG. 7. That is to say, a damper base material is introduced, then impregnated with resin in step 701, and dried in step 702. This resin-impregnated damper base material is hot-pressed by using a die so as to form a corrugation shape in step 703. Thereafter, in a trimming process in step 704, an inner diameter and an outer diameter are punched out by using a die.
The present inventors have proposed, in Japanese Patent Unexamined Publication No. H8-340596, a loudspeaker damper a) being less deteriorated in the basic performance; b) being excellent in water resistance, humidity resistance and heat resistance; c) being excellent in shape-keeping property and less deteriorated in a loudspeaker property after long time of use; and d) in manufacturing process, providing a manufacturing method in which impregnation and molding steps are safe without adversely affecting the working environment and harmful gas is not generated.
The loudspeaker damper proposed in Japanese Patent Unexamined Publication No. H8-340596 includes a cloth composed of fully aromatic polyamide yarns, as a matrix component. The fully aromatic polyamide yarn is a mixed yarn mixed with thermoplastic aromatic polyester fibers having a thermal fusion temperature that is lower than a thermal decomposition temperature by 100° C. or more. In the mixed yarn, fully aromatic polyamide fibers are fixed to each other by fusion of the thermoplastic aromatic polyester fibers. Furthermore, in the mixed yarn, fibers constituting the yarn and fiber surfaces are fixed to each other by a vehicle containing polyester resin. In the cloth, the mixed yarns are fixed to each other at their intersection points by fusion of thermoplastic aromatic polyester fibers and with a vehicle containing polyester resin.
However, recently, in accordance with the digitization of equipment, a loudspeaker used in such equipment is required to have an enlarged dynamic range, that is, high output. However, in the loudspeaker damper shown in Japanese Patent Unexamined Publication No. H8-340596, too much load is applied, so that minimum resonance frequency (F0) of the loudspeaker may be considerably lowered and gap failure may occur because a vibration system of the loudspeaker cannot be sufficiently supported. The present inventors ascertained that such problems are caused by deterioration of the shape-keeping property due to overload to a damper itself during operation of a loudspeaker and that this phenomenon is caused by the reduction in binding strength between a damper base material and resin impregnated into the damper material.
In order to solve such problems, the present inventors have proposed in Japanese Patent Application 2004-196533 that a step of subjecting a base material to surface reforming treatment (corona discharge treatment) before a step of impregnating a base material with resin. The proposition in Japanese Patent Application 2004-196533 makes it possible to improve the wettability of a base material and to improve the conformability between the base material and impregnated resin. Thus, the base material can be impregnated with resin sufficiently, and the binding strength between the base material and the resin can be reinforced.
However, there has been a problem that even a loudspeaker damper described in Japanese Patent Unexamined Publication No. H8-340596 in which the performance is improved by using a thermoplastic aromatic polyester fiber or a loudspeaker damper described in Japanese Patent Application 2004-196533 in which the binding strength between a base material and resin is reinforced by subjecting the base material to corona discharge treatment cannot sufficiently respond to further high output.
That is to say, when a large input is applied to a loudspeaker and a voice coil and a diaphragm oscillate, a damper prevents the oscillation and a resin layer provided on the surface of the damper base material is cracked due to partial interface peeling. Thus, the property is deteriorated.